Here is a link to that crankset. It lists the recommended length and taper. This is where I would start. Generally the crank dictates the spindle length, not the bike. Of course, depending on how your wheels are dishes things can change.

If you have any calipers you can measure the current spindle length even while it is still on the bike. I would build it up with what's on there, check your chainline, and then see if/how badly it is off. From there you can do some simple addition or subtraction to find the right length.

https://www.treefortbikes.com/Sugino...30mm-alloy-RD2